A320 Final App Query
 

Hi all, hope you are all well.

i have a query about final app as I’m slightly confused by the FCOM / FCTM engagement conditions.
one of the engagement conditions is +250ft above the final descent point.

does this mean if the platform is 4k, it will only engage at 4250ft as a minimum
OR
does the 3 degree path (assuming it’s 3 deg in this case) work backwards from the FDP so you can in fact have final app engaged earlier.

the reason why I ask is some skippers brief that they’d like to be level at the platform alt which means from what I’ve seen FINAL APP has engaged at the FDP exactly, where as I’ve also seen it armed during a descent as long as it has been the TO waypoint..

interested in peoples thoughts.. is it really necessary to be level… before arming that is…?

thanks guys hope you all have a nice weekend.

It is NOT necessary to be level, at least not in my experience.

It is possible for FINAL APP to engage before the FDP and if this does happen then it can lead to the aircraft not adhering to Altitude Constraints etc, hence the wanting in OM4:

“If APPR pb is pressed earlier, FINAL APP mode may engage. As a consequence,
resulting speed and altitude management in the FINAL APP may be inappropriate before
FDP”

As a technique it works really well to be level before arming FINAL APP inbound to the FDP, IT gives you the best chance avoiding undesirable behaviours. Arming in the descent inbound to the FDP is not prohibited but does come with risk, if you are aware of the risk and ready to intervene then that’s fine, but why take the risk.